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<title>Swallowing the Semicolon (The C Preprocessor)</title>

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<div class="subsection-level-extent" id="Swallowing-the-Semicolon">
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<h4 class="subsection" id="Swallowing-the-Semicolon-1"><span>3.10.3 Swallowing the Semicolon<a class="copiable-link" href="#Swallowing-the-Semicolon-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-semicolons-_0028after-macro-calls_0029"></a>

<p>Often it is desirable to define a macro that expands into a compound
statement.  Consider, for example, the following macro, that advances a
pointer (the argument <code class="code">p</code> says where to find it) across whitespace
characters:
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define SKIP_SPACES(p, limit)  \
{ char *lim = (limit);         \
  while (p &lt; lim) {            \
    if (*p++ != ' ') {         \
      p--; break; }}}
</pre></div>

<p>Here backslash-newline is used to split the macro definition, which must
be a single logical line, so that it resembles the way such code would
be laid out if not part of a macro definition.
</p>
<p>A call to this macro might be <code class="code">SKIP_SPACES (p, lim)</code>.  Strictly
speaking, the call expands to a compound statement, which is a complete
statement with no need for a semicolon to end it.  However, since it
looks like a function call, it minimizes confusion if you can use it
like a function call, writing a semicolon afterward, as in
<code class="code">SKIP_SPACES (p, lim);</code>
</p>
<p>This can cause trouble before <code class="code">else</code> statements, because the
semicolon is actually a null statement.  Suppose you write
</p>
<div class="example smallexample">
<pre class="example-preformatted">if (*p != 0)
  SKIP_SPACES (p, lim);
else ...
</pre></div>

<p>The presence of two statements&mdash;the compound statement and a null
statement&mdash;in between the <code class="code">if</code> condition and the <code class="code">else</code>
makes invalid C code.
</p>
<p>The definition of the macro <code class="code">SKIP_SPACES</code> can be altered to solve
this problem, using a <code class="code">do &hellip; while</code> statement.  Here is how:
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define SKIP_SPACES(p, limit)     \
do { char *lim = (limit);         \
     while (p &lt; lim) {            \
       if (*p++ != ' ') {         \
         p--; break; }}}          \
while (0)
</pre></div>

<p>Now <code class="code">SKIP_SPACES (p, lim);</code> expands into
</p>
<div class="example smallexample">
<pre class="example-preformatted">do {...} while (0);
</pre></div>

<p>which is one statement.  The loop executes exactly once; most compilers
generate no extra code for it.
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